Heat the oil in a frying pan and add the bacon and onion, Fry until the onion starts to soften and the bacon is almost cooked then add the zucchini and mushrooms and fry until they soften.
Wash the tomatoes and slice the tops off as close to the top as possible.
Scoop out the inside (get out as much as you can) and discard or use for something else.
Now fill the hollow tomatoes with the fried mixture (really stuff it in!)
Top with the grated parmesan cheese.
Place on a baking tray,place in the oven and cook for 15-20 mins